Sitting in his run-down office in theUkrainiancapital Kiev, Yuri Andreyev recalls the events of 25 years ago as if they were yesterday.

As a survivor of the world's worst nuclear accident, he is also surprised that he is still around to tell the tale.

"I absorbed a dose of radiation that should have killed me," says the former Chernobyl engineer, his eyes welling up with tears. "I thought afterwards that it would only be a matter of time before my family had to fend for themselves."

Now 60 and the head of an organisation representing 450,000 people affected by the tragedy, Mr Andreyev's pessimism is understandable.

Many of his co-workers have since died from radiation-related illnesses, and he himself very nearly perished in the explosion itself.

He clocked off his shift at the plant a mere one hour and 23 minutes before a huge blast ripped through its fourth reactor, and was less than two miles away as lethal radioactive matter began to pour out of the stricken building's roof.

Hours later, though, he and three colleagues were back at the plant trying to stop its second reactor blowing up too, water pouring into the control rooms and the alarm system flashing red as they struggled to shut it down.

Thankfully, for the world and for Mr Andreyev, they succeeded.

As Ukraine prepares to mark the 25th anniversary of the Chernobyl disaster next month, its legacy remains as divisive as ever, however. Opponents of nuclear power insist that Chernobyl proved once and for all that the technology is unsafe. They argue that no more nuclear power stations should be built – ever.

"Chernobyl was a warning for the future," said Valery Makarenko, the first Soviet TV reporter on the scene. "It was not just a banal disaster, it was a message that nuclear power is not safe. It is time to think, consider alternatives, and bring the industry under tight international control. Otherwise, humankind will destroy itself."

Proponents of nuclear energy, however, claim the fallout from Chernobyl was actually not as bad as first thought and pin the blame on shoddy Soviet management practices.

Safety standards are much higher now, they point out, and nuclear power is cheap and clean compared to fossil fuels.

As evidence that the effects of radiation are not as bad as critics contend, they cite how wildlife has staged a remarkable comeback in the area around Chernobyl. Audits in the past have shown that the 18-mile exclusion area or "dead zone" around the plant is now home to 66 different species of mammals, including wild boar, wolves, deer, beavers, foxes, lynx and thousands of elk.

Moreover, with oil supplies finite and coal viewed by many as unacceptably polluting, many developing countries, including India and Iran, are pressing ahead with new nuclear facilities. Ironically, it is Russia's atomic energy agency, the successor to the Soviet atomic energy agency that built Chernobyl, that is winning many of the contracts to build these new plants.

Conceived as the largest such plant in the world, the Soviets originally planned to build 12 such reactors at Chernobyl, a peaceful wooded spot that lies 70 miles north of the modern-day capital of Ukraine.

A model Soviet town called Pripyat, built in 1970 to house almost 50,000 plant workers and their families, lay less than two miles from the sprawling power station and completed the Kremlin's nuclear-powered master plan.

That plan ended in the small hours of April 26, 1986, when a "routine experiment" went badly wrong and Chernobyl's fourth reactor exploded, sending a plume of radiation equivalent to 400 Hiroshimas into the night sky.

Many of the 176 staff on duty that night were killed instantly; others would die later in hospital. The reactor core burned for 10 days, and the resultant pollutants - including plutonium isotopes with a half-life of 24,360 years - drifted around the world, raining toxicity down on faraway places such as the lakes of Japan and the glens of Scotland.

The Soviets tried to hush the disaster up and waited almost three days - until the drifting radioactive fallout triggered alarms in Sweden - before publicly acknowledging that an accident had occurred. The reactor-core eventually had to be sealed with a cement mixture, dropped from the air, and a giant steel and concrete sarcophagus erected over it to contain the radiation.

The Soviet Union's mania for secrecy and its desire to save face mean that it is still not known precisely how many people died as a result of the tragedy. Estimates of human fatalities, both direct and indirect, vary wildly, from less than a hundred in the immediate aftermath to tens of thousands in the years that followed.

More widely, an estimated five million people were exposed to potentially hazardous levels of radiation in Ukraine, Belarus and Russia. Doctors claim that cancer rates are far higher than they were before 1986, and that tens of thousands of Ukrainians and people in neighbouring Belarus (worse affected than Ukraine because of the wind direction at the time) may have died prematurely as a result.

"I now understand that health is the main thing in life," laments Ilya Bosakovsky, 72, a former Chernobyl worker, who recalls how soldiers drafted in to help with the clean-up operation refused to do some jobs because they feared being irradiated.

"I was really healthy when I started working at the plant but by the time I had finished my health was shot to pieces. I started getting really bad headaches and my blood pressure rose. "

In the meantime Chernobyl itself and Pripyat, the Soviet model town that has now become a ghostly monument to humankind's incompetence, have become ghoulish tourist attractions open to anyone ready to spend the equivalent of about £100 for a day trip.

The Ukrainian government legalised such tours for the first time in January, and is now developing plans to attract more tourists to the area ahead of the 2012 European football championship in Ukraine.

Ukraine's emergency situations ministry claims that radiation levels in parts of the "dead zone" around Chernobyl are now returning to normal levels, paving the way for the area to be marketed as a tourism destination more widely.

"The Chernobyl zone is not as scary as the whole world thinks," said spokeswoman Yulia Yurshova. "We want to work with big tour operators and attract Western tourists, from whom there is great demand."

The tours are not for the faint-hearted.

Visitors have to sign a waiver, exempting the tour operator from all responsibility in the event that they later suffer radiation-related health problems.

Driven round at breakneck speed, and told not to touch any of the irradiated vegetation or metal structures, "tourists" are invited to briefly inspect the stricken number four reactor from a short distance as the geiger counter guides carry clicks ever higher.

"Let's leave now, it is very dangerous to be here," Vita Polyakova, a tour guide, told a group including The Sunday Telegraph last week. "There are huge holes in the sarcophagus covering the reactor," she added, in a tone that suggested she was not joking.

The most arresting "attraction" is not the ruined plant, however, but the ghost town of Pripyat nearby.

Visitors get to walk through the debris-strewn corridors of its Palace of Culture, admire its crumbling Olympic-sized swimming pool, and wander through the eerily empty classrooms of one of its biggest schools. Hundreds of discarded gas masks litter the floor of the school canteen, Soviet propaganda continues to hang on classroom walls, and children's dolls are scattered about, left where their young owners dropped them in a hurry a quarter of a century ago.

Mr Andreyev, who lived with his family in Pripyat, said it broke his heart to return there a few years ago.

"When I went to have a look at my old flat in the 1990s my heart almost stopped," he remembered. "When I looked at everything that was once so familiar to me I realised how much we had lost."

Alexander Sirota, who now runs an organisation trying to keep the disaster's memory alive, was a 9-year-old school boy in Pripyat at the time. Like most youngsters, he was kept unaware of the true nature of the explosion, and initially had happy memories of being evacuated.

"For us it was an exciting game," he said. "There were soldiers, military helicopters, and firefighters and plenty of time off school."

But with adulthood came knowledge of what had really happened - and the horrifying legacy.

"When people I knew started to die around me, a proper understanding of what it was all about came to me," he said.

He believes that any tours to Chernobyl should be more educational than entertainment-focused, and wants the government to recognise the town of Pripyat as a monument so that it can be preserved.

"If people do not know anything about it history could repeat itself," he warned. "It is impossible to leave Pripyat without being changed."

For Mr Andreyev, however, the real lesson to be had about Chernobyl is not about the future, but about the past. During the Soviet era, he was taught that "Soviet reactors do not explode", especially not ones like Chernobyl, which was originally named in honour of Communist revolutionary Vladimir Lenin.

"I was a Communist and of course an atheist back then," he said, as he downed a tumbler of cognac in one go.